In view of the increase cost of energy, a major consideration in the operating cost of a pyro-processing system is its energy economy. It is also of importance that gas be cleaned prior to venting to stack and not in itself be a source of pollution.
As herein set forth, the kiln of the present system is only supplied with the necessary gas that it requires for its function. With this condition there is less waste of heat from the kiln which in previous arrangements has been wasted. Preheat off-gas at about 700 degrees F., is recirculated and the temperature boosted to the desired temperature. With the present arrangement, it is possible to bypass a larger percentage but a lower quantity of dirty kiln gas with less penalty on the system. This is true because the arrangement provides for recouping 700 degrees F. gas and wasting less high-temperature kiln off-gas. The kiln off-gas becomes the predominant source of the mass of gas required for drying and preheating. This permits all zones to be balanced according to their requirements prior to venting waste gas to the atmosphere. Thus, only gas that cannot be utilized in the system is wasted.
A general object of the present invention is to recycle system off-gas utilizing the heat therein for the system process.
Still another object of the present invention is to boost the temperature of the preheat off-gas and utilize this gas on the grate.
Yet another object of the present invention is to bypass a higher concentration of dirty gas from the kiln by reducing the gas flow through the kiln.
A further object of the present invention is to provide a means whereby it is possible to balance kiln conditions to improve kiln system operating conditions and minimize waste gas temperature and volume.
A still further object of the present invention is to provide for three recycle potentials: (1) preheat gas recycle; (2) high-temperature drying recycle; and, (3) low-temperature drying recycle.